Class D audio amplifiers are well-known and widely recognized to provide energy efficient audio drive of a loudspeaker load by switching a modulated audio signal e.g. pulse width modulated (PWM) or pulse density modulated (PDM), across the loudspeaker load. Class D audio amplifiers typically comprise an H-bridge driver with a pair of output terminals coupled to respective sides or terminals of the loudspeaker load to apply an oppositely phased pulse width modulated or pulse density modulated audio signals across the loudspeaker. Several modulation schemes for pulse width modulated audio signals have been utilized in prior art PWM based class D amplifiers. In so-called AD modulation, the pulse width modulated audio signal at each output terminal or node of the H-bridge is switched between, or toggles between, two different levels in opposite phase. The two different levels typically correspond to the upper and lower power supply rails, respectively, such as the positive and negative DC voltage supply rails of the output stage. In so-called BD modulation, the pulse width modulated signal across the loudspeaker load is alternatingly switched between three levels of which two may correspond to the above-mentioned upper and lower DC voltage supplies while the third level is zero. The latter is obtained by simultaneously pulling both terminals of the loudspeaker load to one of the DC voltage supply rails. In multi-level PWM modulation as described in the applicant's co-pending patent application PCT/EP2011/068873, a third supply voltage level, often a set to a mid-supply level between the positive and negative DC supply rails, is applied to output node(s) of the output driver such that for example a 3-level or 5-level pulse width modulated signal can be applied across the loudspeaker load by an appropriately configured output driver.
However, there is a continued need in the art for providing effective overload, e.g. overcurrent, protection of the power transistors of the output stage while keeping power consumption of the overload protection circuitry to a minimum. It is particularly beneficial to reduce power consumption of the overload protection circuitry of class D audio amplifiers to a minimum during quiescent operation and at small audio output levels to prolong battery lifetime of portable audio equipment, reduce heat dissipation etc. At quiescent operation and small output signal levels, the power consumption of the overload protection circuitry can represent a significant portion of the total power consumption of the class D audio amplifiers and therefore render their overall efficiency sub-optimal under these operating conditions unless protection circuitry power consumption is reduced.